Fuels, from firewood to coal to 98RON PULP represent one thing: stored energy. The energy sits dormant in the chemical bonds between atoms in complex molecules. Good fuels store a lot of easily accessible energy in a small volume, so in that respect petrol is better fuel than firewood. Diesel is likewise better than coal, which is why (coal-fired) steam locomotives were superseded.
The most common elements in fuels are carbon, hydrogen and oxygen. Because the taxonomy of science is unimaginative, fuels containing only hydrogen and carbon are called ‘hydrocarbons’. These can be gases like LPG (a.k.a. propane; three carbon atoms and eight hydrogens) or liquids like petrol (petrol approximates ‘octane’; eight carbons and 18 hydrogens).
Fuels containing all three elements are called carbohydrates – sugars and starches, as well as the cellulose in firewood and paper. Alcohols like ethanol are hydrocarbons with only a little oxygen built in, slightly different to sugars and starches.